An imaging charge-coupled device (CCD) is a silicon-based microelectronic device comprised of an array of closely spaced metal-oxide semiconductor (MOS) diodes that convert incident light (photons) into electronic charge. Using a lens, an image is focused onto the CCD pixel array. Incident photons are absorbed in a silicon substrate, creating electron-hole pairs. The photo-generated electrons are attracted to a positively biased CCD gate electrode which then traps the electrons (charge) within a potential well formed under the CCD gate electrode, with the trapped charge density (number of electrons) proportional to the intensity of the incident light. To read out the captured charge-based image, a sequence of voltage pulses are applied to the gate electrodes to linearly transfer the trapped charge packet along individual columns and into an external electrometer that then converts the collected charge into a voltage proportional to the gray scale of the incident image. The individual columns are electrically isolated from each other using P+ doped (channel stop) regions.
Recent consumer, industrial, and Government demand for ever higher resolution digital cameras as well as cell phones with integrated cameras (i.e., more mega-pixels) has rapidly forced a shrink in the size of imaging pixels to provide the required increased resolution while still maintaining comparable or even smaller die sizes to keep manufacturing costs down. Avoiding degradation in digital image quality becomes more challenging as pixel sizes are aggressively shrunk due to the fact that less pixel (silicon) area is available to both collect and transfer photo-generated charge. Not surprisingly and with no changes to the CCD pixel structure, reducing the size of the pixel results in less area available to store charge. The effect is a reduction in the CCD dynamic range, which results in degraded image quality for an equivalent image capture interval. While the CCD pixel charge handling capacity sets an upper limit on the maximum signal that can be captured without degradation, the dark or leakage current sets the lower limit on the minimum detectable signal (dimmest image) within the sample period. In CCD's, one of the dominant sources of dark current are interface states present at the silicon to gate dielectric (oxide) interface.